stuttering problem

[furrygreg]

The bike is a 1978 XS650SE. Had 8500 original miles prior to rebuild. Went through the entire engine last year while doing the build. Cylinders only required honing, no oversize. Crank & cam are OEM. All bearings & bushings measured well within limits. Besides replacing normal wear & tear parts such as cam chain guides, the only internal engine mod was to upgrade to Mike's new 5th gear with the extra tooth. We installed a Boyer Brandsden ignition system and a Mike's PMA charging system. The carbs are Mikuni VM34's with cone filters, and the exhaust is a MAC two into one megaphone. The bike runs very strong and never gives me a problem. I can kick it from cold if I want even though i am running a Boyer Brandsden electronic ignition and not the PAMCO system. The charging system puts out 14 to 14.5 volts depending on if I am running at idle or 2000 RPM's or more. The timing is set up per Boyer instructions to 12 degrees before TDC. The carbs currently have a 159 series P5 needle jet, 6F9 needle, 210 series 20 pilot jet, & a 4/042 series 180 main jet. The needle clip is set to the center position. The elevation at Tucson Arizona is about 2500 FT. The fuel mixture screws are set at 3/4 turn out from bottom each. Both were adjusted using the dead cylinder method. The idle screws are set so that the engine idles at 1200 RPM's with the engine warm. My son and i have put over 2500 miles on the bike since the build was completed last January. This was our first bike project together, and because of all the info and parts that are available out there, the bike turned out very well. We get a lot of people walking up the bike wanting to know about it. The only problem I have is a nagging stuttering problem when running around 80 MPH at 4500 RPM. If I go above or below those parameters, the bike is smooth as silk. Sometimes I get a little stuttering when accelerating from a stop if I don't "roll" the throttle. Reading the tuning manual, I should probably go down one size main jet. Should i adjust the needle clip or the pilot jet as well? I have topped the bike out to about 110 MPH on a long open road before it ran out of steam. I am very open to experienced advice. If I can get this last problem solved, i will be one happy mid-life crisis dad. Thanks for the help.

 

[retiredgentleman]

Click on "Tech" at top of page. Go down to "Carbs Manuals, guides" and click on that. In there you will find a section on VM carbs. Should be good to read through that very carefully.

 

[TwoManyXS1Bs]

You could do a plug read at the problem zone. Start with new plugs, go to your cruise 'problem' speed, avoid other speeds, stay at that speed for awhile, then pull-in clutch, shut it off, coast to a stop (for cooling). Do roadside plug reading...

 

[furrygreg]

I actually printed up the Mikuni VM34 carb manual and followed it. It was a very good guide to reference to get me to where I am at right now. I will try the method of installing new plugs and then running the bike in the stuttering range for a little while, then killing the ignition so I can pull over and look at the plugs. I'll bet I will find that I am running rich and will need to drop one size down on the main jet. I should be able to leave everything else as is as the other components do not affect higher RPM range operations.

 

[TwoManyXS1Bs]

I'm thinking that 80mph is still in the 'needle' range. The plug read will tell you rich or lean. Might as well do diaphram test too.

 

[furrygreg]

Mikuni VM34 carbs don't have diaphragms. I went with them because the original B38 CV carbs were always giving me grief. I will do the plug test this weekend.

 

[Bloody D]

i just tossed my bs38's in the trash today. i think id rather push my bike than deal with them any longer. Anyway i was just reafing up on the vm's and 2 different post i have read have the sme problem with the dead spot at mid range. Both had 2-1 pipes, hughs blog had the post. problem went away with dual pipes

 

[5twins]

Yes, some of the 2-1s can be difficult, even impossible to tune for. Many weren't designed to achieve best performance but rather to bolt on and fit the bike. Important tuning aspects like pipe and collector lengths and diameters were not applied. Twins run best with a 2 into 2 utilizing long, gently tapered megaphones. Switch to something like that and I'll bet your stumbling problems disappear.